The radio frequency ("RF") signals used to transmit information over cable television ("CATV") distribution systems are subject to several types of undesirable noise. For example, snow noise, composite second order ("CSO") noise, and composite triple beat ("CTB") noise are three well-known sources of noise typically found in RF signals transmitted over CATV distribution systems. In efforts to improve service, CATV service providers perform noise measurements to quantify the performance level of the network, or a portion thereof.
Such CATV noise measurements typically include measurements of particular types of noise, i.e., CTB or CSO noise, for diagnosis of CATV distribution system problems. If a particular type of noise is determined to be unusually high, while other types of noise are determined to be at normal levels, the CATV service provider can more readily determine the appropriate corrective action to be taken. For example, if the CSO noise level is measured to be unusually high, then the service provider might try a first set of corrective actions. If, however, the CTB noise is measured to be unusually high, then the service provider might try a second set of corrective actions. Thus, knowledge of both the presence and type of noise is important in CATV system diagnostics.
Moreover, federal regulations require that CATV service providers perform a plurality of noise measurements on a regular basis, including, for example, a CTB noise measurement. CTB noise is caused by concentrations of triple beat signals that occur near channel frequencies in cable television systems. As is well known in the art, the triple beat signals are caused by the beating of signals from three other CATV channels. The resulting CTB causes interference, thereby reducing signal quality. In United States CATV systems, CTB noise is generally concentrated within 15 Khz of the visual carrier frequency of select channels in a CATV broadband signal. By contrast, CSO noise is concentrated near 0.75 MHz and 1.25 MHz above the carrier frequency and near 0.75 MHz and 1.25 MHz below the carrier frequency. Snow noise, moreover, is more or less evenly distributed across the entire CATV broadband signal spectrum. Because CTB noise is concentrated so close to the channel frequency, CTB noise is often more difficult to measure than snow noise or CSO noise.
Because the CTB noise energy is located near the carrier frequency, each channel historically had to be taken out of service in order to facilitate the CTB measurement. Specifically, a technician would first attach test measurement equipment to a remote site on the cable distribution network and measure the carrier level of the CATV channel while the channel was in-service (or producing a test pattern). The technician would then cause the CATV transmitter to take a channel out of service, or in other words, remove the carrier from that channel. Once the carrier was removed, the technician would perform a signal strength or power measurement within a band of +/-15 Khz from the carrier frequency. Because the carrier signal had been removed from the channel, the measured signal power found within +/-15 Khz of the carrier frequency constituted the CTB noise level. The ratio of the noise power level to the carrier power level constituted the CTB noise measurement. Once the CTB noise measurement was completed, the CATV channel could be placed in-service again.
The above described method has at least one severe drawback. Specifically, removing the carrier signal causes interruptions in CATV service, which is undesirable for several reasons. Interruptions in CATV service often lead to customer dissatisfaction and customer complaints.
To reduce service interruptions, U.S. Pat. No. 5,617,137 to Whitlow shows an in-service CTB noise measurement system that measures the CTB noise level of an active television signal. The Whitlow method first demodulates the received television signal to produce a baseband television signal including the noise signal. Then, the baseband signal information is removed, leaving only the noise signal. An RMS type measurement is then performed on the noise signal. To obtain a CTB noise measurement, the baseband signal is first filtered such that it contains primarily only frequencies in the CTB noise energy spectrum. Accordingly, the RMS measurement yields, in theory, on the CTB noise.
One drawback of the Whitlow method described above is the method in which the baseband signal information is removed to produce the noise signal alone. According to the Whitlow method, a portion of one frame of baseband signal information is subtracted from the same portion of a subsequent frame of baseband signal information. The portion of the baseband signal information that is used is a repeating pattern known as the vertical synchronization interval. The drawback of the Whitlow method relates to the subtraction of one frame from a subsequent frame to eliminate the baseband signal information. Specifically, such subtraction also subtracts the CTB noise of one frame from the CTB noise of the subsequent frame. While the CTB noise will often vary greatly from frame to frame, the subtraction of one frame of CTB noise from another frame of CTB noise will not produce a reliable CTB noise estimate.
In addition, the Whitlow method requires analog demodulation devices and filtering devices which undesirably add to the cost of a device designed to carry out the method. In particular, the Whitlow method requires a video demodulator, a low pass filter for isolating the CTB noise, and a band pass filter for generating non-CTB noise floor reference values. Such devices not only add to the cost of the device, but they also occupy valuable circuit board space, and increase the overall size, weight and energy consumption of the device. The space and energy requirements are especially important due to the intended portable nature of the device.
Finally, another drawback to the Whitlow method is that CTB noise measurements are subject to errors contributed by hum noise. Hum noise is 60 Hz noise produced by amplifiers in the CATV distribution system. Hum noise results from imperfect isolation of the 60 Hz power line signal in the power supplies of the amplifiers. It is noted that Whitlow provides some attenuation of hum noise through its subtraction of one video frame from another video frame. Because video frames occur at a frequency of 59.97 times per second, much of the 60 Hz noise is filtered by the subtraction of information in subsequent frames. However, because hum noise can be substantial compared to CTB noise, even a small difference between the frame rate and the hum noise frequency can result in insufficient filtering to remove hum noise from the CTB noise measurement.
As a result, there exists a need for more energy, cost and space efficient in-service noise testing method for use in CATV distribution systems. Moreover, there exists a need for such an in-service noise testing method that has increased accuracy.